In a gas turbine system a gaseous or liquid fuel, for example natural gas or crude oil, is mixed with compressed air and combusted. The pressurized combustion exhaust gases are supplied to the turbine of the gas turbine system as the working medium. The working medium sets the turbines under expansion into rotation, with thermal energy being converted into mechanical work, i.e. the rotation of the turbine shaft. When the expanded working medium is discharged from the gas turbine system said medium typically still has a temperature of 500-600° Celsius.
In a gas and steam turbine system the expanded working medium, also called flue gas, from the gas turbine system is used to generate steam for driving a steam turbine. Toward that end the working medium is supplied to a heat recovery steam generator connected downstream of the gas turbine system on the exhaust gas side, in which steam generator heating surfaces are arranged in the form of pipes or pipe bundles. Said heating surfaces are in turn connected into a water-steam cycle of the steam turbine system which has at least one, but mostly a plurality of pressure stages. The pressure stages differ from one another in that the water supplied to the heating surface for the purpose of generating steam has different pressure levels. A gas and steam turbine system comprising a water-steam cycle having only one pressure stage is described in DE 197 36 888 A1, and such a system comprising three pressure stages, namely a high-pressure stage, a medium-pressure stage and a low-pressure stage, is described in DE 100 04 187 C1.
Currently, in order to start a gas and steam turbine system, the gas turbine system is usually started up and the expanded working medium is supplied to the heat recovery steam generator of the steam turbine system. Initially, however, the steam generated in the heat recovery steam generator is not fed to the turbine part of the steam turbine system, but is directed past the turbine via diverter stations and supplied directly to a condenser which condenses the steam to water. The condensate is then supplied to the steam generator again as feedwater. In many embodiment variants of gas and steam turbine systems the diverted steam is also conveyed to the atmosphere.
The steam turbine is only switched into the cycle when certain steam parameters in the steam lines of the water-steam cycle or in the steam lines leading to the turbine part of the gas turbine system, for example certain steam pressures and temperatures, are complied with. Complying with said steam parameters is designed to keep potential stresses in thick-walled components at a low level.
After the startup of the gas turbine system there is a power increase which leads to an increase in pressure in the steam system. The load gradient at which the gas turbine system is started up, i.e. the power increase of the gas turbine system per time unit, is critically dependent on the implementation and mode of construction of the heat recovery steam generator as well as on the structural limitations within the steam turbine. As the gas turbine load and consequently the temperature or, as the case may be, the volume flow rate of the exhaust gas emitted from the gas turbine system increase, the steam temperature and the pressure in the steam system are also increased.
Before the steam turbine starts up, the gas turbine is typically kept at a specific partial load until stationary states have come about in the gas turbine system and in the steam system. As soon as stable steam production has been reached, the steam contained in the steam system is channeled to the steam turbine, thereby accelerating the steam turbine. The turbine speed is then increased to nominal speed. Following synchronization of the generator coupled to the steam turbine with the power supply system, or in the case of single-shaft systems, following the engagement of the overrunning clutch, the steam turbine is subjected to further load as a result of an increase in the steam supply. At the same time the diverter stations close more and more in order to keep the steam pressure roughly constant and minimize level fluctuations in the heat recovery steam generator.
As soon as the diverter stations are closed and the steam produced in the heat recovery steam generator is channeled in its entirety to the steam turbine, a further increase in the gas turbine power output takes place when there is a higher power requirement on the part of the system which is now operating in the gas and steam turbine mode.
By definition, the startup operation of a gas and steam turbine system is terminated only when the gas turbine has reached the base load and all diverter stations are closed.